1. Field of the Invention
This invention relates to a photoacoustic imaging method for nondestructively obtaining planar images of an object cut at any desired depth beneath its surface.
2. Prior Art Statement
When an object is illuminated with modulated light, the light absorbed by the object is intermittently converted to heat, thus generating sound caused by the accompanying thermal strain.
A photoacoustic signal obtained by extracting this sound as a signal varies with respect to physical properties of the object including its absorptance of light, the speed of conduction of heat through the object and various other properties, so by using this photoacoustic effect, the interior of the object can be observed in a nondestructive manner.
Microscopes which exploit the photoacoustic effect in this manner, e.g. U.S. Pat. No. 4,255,971, have been disclosed, and are being employed in the nondestructive inspection of the interior of products which cannot be seen with visible light.
The resolution of these photoacoustic microscopes in the direction of the two-dimensional surface illuminated by modulated light is dependent on the spot diameter of the incident light. Resolution in the depth direction of the illuminated surface is dependent on the thermal diffusion length, or the distance over which heat travels through the object during one cycle of the modulated light. Therefore, increasing the modulation frequency decreases the thermal diffusion length, so as shown in FIG. 1(a), a photoacoustic signal is generated from a region 3a near the surface of an object 2 illuminated by modulated light 1, and thus this region can be observed, but a defect area 4 located deeper within the object and outside of the signal-generating region cannot be observed. On the other hand, if the modulation frequency is decreased, the thermal diffusion length increases so as shown in FIG. 1(b), a photoacoustic signal is generated from a wide region 3b which includes areas located both near the surface and deep under the surface, so the defect area 4 can be observed even if it is present deep within the object. However in this case, photoacoustic signals from areas which need not be observed (shallow areas near the surface of the object) become noise which is incorporated into the detected photoacoustic signal, greatly affecting it so that a clear image of the defect area 4 only cannot be obtained.
Therefore, in the present situation, an appropriate modulation frequency must be selected depending on the type of object to be observed.
The object of this invention is to supply a photoacoustic imaging method through which one is able to nondestructively obtain clear planar images at any desired depth from the surface of an object.